The present invention relates generally to printing systems and more particularly to tandem printing systems for printing variable information using two or more printing stations and including a paper-position correction mechanism.
Tandem printing systems, that is printing systems with two or more printing engines, are well known, both for duplex printing and for multi-colour, single-side printing, with each colour being printed with a different one of a tandem series of printing engines. Such systems are known both for conventional and electronic printing. However, such systems depend on a very accurate transmission and edge alignment of the paper.
In systems of rollers and belts, a mismatch in transmission and edge alignment may occur for various reasons. The rollers may be slightly off centre, the paper may slip or creep off is its hold, there may be diameter variations, for example, because of different paper thickness and the belts may stretch as a result of heat or age. To achieve the accuracy required in tandem printing, these misalignments must be corrected.
One aspect of some preferred embodiments of the invention relates to a tandem printer with a mechanism for fine paper-position correction.
Preferably, the tandem printer has a motive system comprising a rotatable element which rotates at a given rotation rate and on which the paper is mounted, wherein a step angular displacement to the rotatable element brings the paper into alignment, without changing the rotation rate of the rotatable element.
Preferably, a paper sensor measures the position of the paper. The measurements are reported to a controller which applies the step, angular displacement to the rotatable element, in addition to the continuous rotation, responsive to the measurements of the sensor.
Preferably, the motive system also comprises a flexible strip, for example, a timing belt, which transfers motion at a constant rotation rate from a driving roller to the rotatable element, wherein a step displacement to the flexible strip induces the step angular displacement to the rotatable element. The axis of the flexible strip is defined as the line connecting the centre of the rotatable element and the driving roller.
Preferably, the step displacement of the flexible strip is provided by linear motion of two pulleys located upstream and downstream of the rotatable element, wherein as one pulley presses onto the flexible strip, requiring slack, the other pulley pulls away from the flexible strip, releasing slack. The transfer of flexible-strip slack from one pulley to the other provides the step displacement of the flexible strip.
Preferably, the tandem printer comprises means for providing the required linear motion to the pulleys.
In some preferred embodiments of the invention, means for providing linear motion to the pulleys comprises a rod having two edges on which the two pulleys are mounted. The rod is preferably situated perpendicular to the axis of the flexible strip, with one pulley, at one edge, pressing against the flexible strip upstream of the rotatable element, at a first point, and the other pulley, at the other edge, pressing against the flexible strip downstream of the rotatable element, at a second point. Movement of the rod up and down in a direction generally, perpendicular to the axis of the flexible strip provides the required linear motion to the pulleys.
Alternatively, means for providing linear motion to the pulleys comprises a shaft on which one pulley is mounted, pressing against the flexible strip at a first point and a spring-loaded device (for example, a spring-loaded piston-cylinder device) on which the other pulley is mounted, in partial compression, pressing against the flexible strip at a second point, wherein the first and second points are upstream and downstream of the rotatable element, in any order, reasonably far from any rotating elements associated with the flexible strip. Linear movement of the shaft provides the linear motion to the pulley mounted on it. The response of the spring-loaded device to the release or demand in slack provides the motion of the other pulley.
Preferably, a stepper motor provides the motion for the pulleys. In some preferred embodiments an eccentric shaft is used to convert the motor motion to linear motion. Alternatively, any of a slider-crank mechanism, a piston-cylinder mechanism, or a turning-screw mechanism may be used. Alternatively still, any other method of providing linear motion, known to persons versed in kinematics, may be used.
Preferably, the motor is activated by a controller which determines when the paper is out of alignment and the magnitude and direction of the misalignment.
In some preferred embodiments, the section of the flexible strip adjacent to the first point and the section of the flexible strip adjacent to the second point are parallel. For small displacements, the step angular displacement of the rotatable element is symmetric for upward and downward linear displacements of the pulleys.
Preferably, the step angular displacement of the rotatable element is given as a function (which may be empirical) of the linear displacement of the pulleys. Alternatively, a lookup table is used.
In some preferred embodiments of the invention, the tandem printer comprises a duplex printer for printing on both sides of paper while inverting it. Alternatively, the tandem printer comprises a multicolour printer of single side printing, with each colour being printed with a different one of the tandem series of printing engines.
In some preferred embodiments of the invention, the tandem printer comprises any conventional printer, such as a printer which prints directly from plates. Alternatively, the tandem printer comprises any of a lithographic printer, an electrostatic printer, or an electronic printer.
There is thus provided, in accordance with a preferred embodiment of the invention a tandem printer with a mechanism for fine substrate-position correction, comprising:
a first printing station;
a second printing station;
a rotatable element, rotating at a given rotation rate, that receives the substrate after printing thereon by the first printing station and transfers the substrate toward the second printing station;
a sensor which measures the position of an edge of the substrate during its transfer from the first printing station to the second printing station; and
a controller, which applies a corrective step change in angular position of the rotatable element responsive to the measurements of the sensor, without changing the general rotation rate of the rotatable element.
Preferably, the sensor which measures the position of an edge of the substrate is situated on the rotatable element.
Alternatively, the sensor which measures the position of an edge of the substrate is adjacent to the rotatable element.
In a preferred embodiment of the invention, the tandem printer also comprises:
a transfer system which transfers the substrate from the first printing station to the second printing station, in which the rotatable element is comprised;
the transfer system further comprising:
a flexible strip, travelling at a given rate and providing motion to the rotatable element, wherein a corrective step displacement of the flexible strip induces the corrective step change in angular position of the rotatable element.
Preferably, the flexible strip rotates at a constant rate.
Preferably, the flexible strip is a timing belt.
In a preferred embodiment of the invention, the tandem printer also comprises at least one pulley that provides the corrective step displacement of the flexible strip.
In a preferred embodiment of the invention, the at least one pulley comprises:
two pulleys, situated along the flexible strip, one upstream and one downstream of the rotatable element, said pulleys pressing into the flexible strip at a first point and a second point, respectively, wherein when pressure of one pulley is partially released, the other pulley takes up the thus produced slack, providing the corrective step displacement of the flexible strip.
In a preferred embodiment of the invention, the tandem printer also comprises a rod, comprising two points, to which the two pulleys are attached, one at each edge, wherein linear movement of the rod provides the motion of the pulleys into and away from the flexible strip.
Preferably, the tandem printer also includes a motion provider for the rod, comprising:
an eccentric shaft to which the rod is attached; and
a motor which provides motion to the eccentric shaft,
wherein the motor is activated by the controller.
Alternatively, the tandem printer includes a motion provider for the rod, comprising:
a slider-crank mechanism, wherein the rod is attached to the slider and moves in the same direction as the slider; and
a motor which provides motion to the slider-crank mechanism,
wherein the motor is activated by the controller.
Alternatively, the tandem printer includes a motion provider for the rod, comprising:
a piston-cylinder mechanism, wherein the rod is attached to the piston and moves in the same direction as the piston; and
a motor which provides motion to the piston-cylinder mechanism,
wherein the motor is activated by the controller.
Alternatively still, the tandem printer includes a motion provider for the rod, comprising:
a turning-screw mechanism, wherein the rod is attached to the screw and moves in the same direction as the screw; and
a motor which provides motion to the turning-screw mechanism,
wherein the motor is activated by the controller.
Preferably, the motor is a stepper motor.
In another preferred embodiment of the invention, the tandem printer also comprises:
a shaft on which one of the two pulleys is mounted, pressing against the flexible strip at a first point; and
a resilient device on which the other pulley is mounted, resiliently pressing against the flexible strip at a second point,
wherein linear movement of the shaft provides motion of the pulley at the first point, and the response of the resilient device to release or demand in slack provides motion of the pulley at the second point.
Preferably, the shaft is an eccentric shaft and including:
a motor which provides motion to the eccentric shaft,
wherein the motor is activated by the controller.
Preferably, the tandem printer also includes a motion provider for the shaft, comprising:
a slider-crank mechanism, wherein the shaft is connected to the slider and moves in the same direction as the slider, and
a motor which provides motion to the slider-crank mechanism,
wherein the motor is activated by the controller.
Alternatively, the tandem printer also includes a motion provider for the shaft, comprising:
a piston-cylinder mechanism, wherein the shaft is connected to the piston and moves in the same direction as the piston; and
a motor which provides motion to the piston-cylinder mechanism,
wherein the motor is activated by the controller.
Alternatively still, the tandem printer also includes a motion provider for the shaft, comprising:
a turning-screw mechanism, wherein the shaft is connected to the screw and moves in the same direction as the screw; and
a motor which provides motion to the turning-screw mechanism,
wherein the motor is activated by the controller.
Preferably, the motor is a stepper motor.
In a preferred embodiment of the invention, the two pulleys are substantially identical.
In a preferred embodiment of the invention, the section of the flexible strip adjacent to the first point and a section of the flexible strip adjacent to the second point are parallel to each other.
In a preferred embodiment of the invention, the tandem printer comprises a duplex printer for printing on both sides of the paper while inverting it.
Alternatively, the tandem printer comprises a multicolour printer of single-side printing, with each colour being printed with a different one of the tandem series of printing engines.
In a preferred embodiment of the invention, the tandem printer is a conventional printer which uses plates.
Alternatively, the tandem printer is an electrostatic printer.
Alternatively, the tandem printer is an electronic printer.
Alternatively, the tandem printer is a lithographic printer.
In a preferred embodiment of the invention, a multi-engine printer with a mechanism for fine substrate-position correction, is provided, comprising at least 3 printing engines, wherein each adjacent pair of printing engines comprises a first printing station and a second printing station, of the tandem printer described herein.
There is further provided, in accordance with a preferred embodiment of the invention a tandem printing method while applying a fine positional correction to a substrate, comprising:
printing on a substrate by a first printing station;
transferring the substrate from the first printing station toward a second printing station, comprising:
mounting the substrate on a rotatable element of a substrate-transfer system; and
moving the substrate by rotating the rotatable element at a given rotation rate;
measuring the angular position of an edge of the substrate on the rotatable element; and
applying a step angular displacement to the rotatable element, responsive to the measurement, without changing the rotation rate of the rotatable element.
Preferably, rotating the rotatable element at a given rotation rate comprises rotating the rotatable element by a flexible strip; and
applying the step angular displacement to the rotatable element comprises applying a step displacement to the flexible strip.
Preferably, applying the step displacement to the flexible strip comprises any of a positive and negative step displacements to the flexible strip, thus inducing any of a clockwise and a counterclockwise step angular displacements to the rotatable element.
Preferably, applying the positive step displacement to the flexible strip comprises:
releasing flexible strip slack upstream of the rotatable element; and
taking up flexible strip slack downstream of the rotatable element; and
applying the negative step displacement to the flexible strip comprises:
releasing flexible strip slack downstream of the rotatable element; and
taking up flexible strip slack upstream of the rotatable element.
Preferably, the method also includes using a lookup table to calculate a necessary step displacement of the flexible strip in order to achieve a desired step angular displacement of the rotatable element